The present invention relates to a compound roll comprising a shell portion having excellent wear resistance and resistance to surface roughening and a tough core portion, and a method of producing such a compound roll by a centrifugal casting method, particularly to a compound roll having a shell portion having a fine and uniform metal structure and a method of producing such a compound roll.
In a roll used for hot rolling or cold rolling of steel materials, it is required that a shell portion, which is brought into direct contact with materials to be rolled, has a uniform cast structure and excellent wear resistance, resistance to surface roughening and crack resistance. To meet this requirement, it is effective to form the shell portion by a centrifugal casting method, and the production of compound rolls having shell portions and core portions is widely conducted. In the centrifugal casting method, a melt for a shell portion is usually introduced into a hollow cylindrical mold rotatable around its longitudinal axis at a high speed, and solidified in the mold.
In this case, since the melt is rapidly cooled by contact with an inner surface of the mold usually made of steel, the shell portion of the compound roll has a fine metal structure. However, as a distance between the inner surface of the mold and the melt to be solidified becomes larger, the cooling speed of the melt for the shell portion decreases, and the temperature gradient of the melt becomes smaller. As a result, the metal structure of the shell portion becomes coarser, so that various properties required for the shell portion such as a wear resistance, etc. are deteriorated. To solve this problem, it is considered to be effective that the cooling speed of the melt for the shell portion is increased, and it is necessary to make the cooling speed of the melt as even as possible in any portion along the radial direction of the shell portion.
To increase the cooling speed of the melt for the shell portion, proposals were made to cool the mold by water, and to spray the melt onto an inner surface of the mold (Japanese Patent Laid-Open No. 1-254363). Also, to avoid undesirable segregation and other defects generated in the shell portion and to improve the uniformity of the shell portion, it was proposed to move the point of pouring the melt into the mold in the centrifugal casting method (Japanese Patent Publication No. 50-33021). Further, research has been conducted on the materials for the shell portion. At present, the shell portion produced by the centrifugal casting method is mainly made of a high-alloy cast iron, a high-chromium cast iron, a high-chromium cast steel, etc.
Among them, in the case of a roll mainly used on a rear side in a tandem mill, which has a shell portion made of a high-alloy cast iron, the non-uniformity of the cast metal structure of the shell portion due to the existence of coarse precipitated grains and segregation leads to a poorer wear resistance, resulting in the increase of roll consumption per a unit weight of a material to be rolled and poorer quality of the rolled material.
Since high quality is increasingly required for rolled steel sheets recently, high requirements are imposed on the roll. Therefore, the shell portion of the compound roll is required to have an increasingly finer metal structure with higher uniformity.
The high-alloy cast iron is a material in which graphite particles are inherently likely to be precipitated. Accordingly, in the case of forming the shell portion from the high-alloy cast iron, a surface portion of the shell portion not only has a fine metal structure but also contains fine graphite particles and a fine carbide phase by the rapid cooling action of the mold. However, since the rapid cooling action of the mold decreases inside the shell portion, the metal structure becomes coarser and the amount of graphite particles precipitated increases while the amount of the carbide phase decreases. As a result, in the deep area of the shell portion, which is to be exposed by several times of machining, it shows poor resistance to wear and surface roughening.
In the case of forming the shell portion by a centrifugal casting method, there is also a problem that the shell portion inevitably contains cast defects and non-uniformity of the metal structure. Since the cooling speed (temperature gradient) of the shell portion is smaller in the inside than in the surface portion, it is difficult for gas, dissolved elements, impurities, etc. in the melt for the shell portion to escape toward an inside cavity into which the melt is poured. Accordingly, these components are trapped in the process of solidifying the melt, resulting in the segregation, coarse metal structure, gas defects, etc.